Known in the prior art is a freezing-and-thawing plant (the USSR Inventor's Certificate No. 704643 Class C 02 C 3/00, published 1979) comprising a crystallizer in the form of a horizontal cylinder provided with a sediment removal unit having a knife, and a melter which comprises a thawing tank with an agitator and a condensing heat exchanger in the capacity of a water heater.
The known plant is insufficiently reliable in operation due to the employment of gland seals on the cylinder shaft, nonuniform distribution of refrigerant over the internal surface of the cylinder, incomplete settling of the solids contained in the sediment due to turbulization of water by the agitator.
Another prior art plant for treating the sediment of natural and waste waters (the USSR Inventor's Certificate No. 582211, Class C 02 3/00, published 1977) comprises a crystallizer, a melter arranged right under said crystallizer, a condensing heat exchanger, a compression refrigerating unit communicating with the crystallizer and, via the condensing heat exchanger, with the melter which latter is in communication with the sediment dehydration apparatus. The crystallizer incorporates an evaporator comprising two coaxially-arranged vertical cylinders interconnected at the ends by a base and a cover. The crystallizer comprises a unit for application of sediment to the vertical walls of the evaporator cylinders, said walls functioning as heat-exchanging surfaces. The sediment application unit is provided with chambers which have sealing elements. The unit also contains a sediment removal unit provided with one cutting tool in the form of a knife or a cutter. The sediment application and removal units are secured on the drive shaft and rotate on a planetary principle relative to the heat-exchanging surfaces of the evaporator cylinders. The crystallizer evaporator incorporates a refrigerant feed unit and a refrigerant discharge unit. The sediment freezes on the heat-exchanging surfaces due to boiling of refrigerant between the cylinders. The melter of the plant comprises a vessel with a conical bottom, said vessel having a cylindrical partition inside whose external surface carries a tubular condensing heat exchanger.
The known plant is little reliable in operation through the following reasons. Owing to its thicotropic properties, the sediment contained in the chamber of the sediment application unit partly restores its structure which was disturbed in the course of transportation. The structure-restoring process in the sediment goes on nonuniformly which results in formation of flakes of variable viscosity and size. Therefore, the heat-exchanging surface becomes coated with an irregular layer of frozen sediment which causes nonuniform contact of chamber sealing elements and leakage of sediment from said chambers. If the sediment application unit stops for some reason, the chambers freeze to the heat-exchanging surfaces of the evaporator cylinders. The knife or cutter is installed at a certain distance from the heat-exchanging surfaces so that part of the frozen sediment fails to be cut off. This results in the increased local thermal resistance, incomplete freezing of the next sediment layer and nonuniform contact of the chambers of the sediment application unit with the heat-exchanging surfaces.
Improvement of the water-yielding properties of sediment is achieved only under preset thermal conditions in the course of freezing. The process of heat exchange is influenced by thermal hydrodynamic parameters which latter depend on the total heat-transfer coefficient, the coefficient of convective heat transfer from the refrigerant and refrigerant, and by the temperature of sediment and refrigerant. Deviation of these parameters from optimum values impairs the reliability of operation of the entire plant. In the plant of the present invention the thermal hydrodynamic parameters may deviate due to the following reasons. The evaporator becomes operational only being fully filled with liquid refrigerant. A considerable height of the column of liquid refrigerant influences the values of coefficients of heat transfer from refrigerant to the cylinder wall said values being local throughout the height of the evaporator and thus causes different intensity of heat exchange between the sediment and refrigerant. Different coefficients of convective heat transfer are also observed during refrigerant feed to a certain portion of the evaporator since its further spreading inside the interwall space of the evaporator proceeds with a varying intensity. The change in the coefficient of convective heat transfer from the side of refigerant impairs the reliability of the plant. The temperature of the sourse sediment may vary within considerable limits even during a day which changes the conditions of heat exchange and thermal load of the evaporator. Violation of the heat exchange conditions and coefficient of convective heat transfer from the side of the sediment reduces the efficiency of sediment freezing which likewise impairs the reliability of the plan operation.
The plant of the present invention has no provision for changing the freezing conditions, the rotation speed of the sediment application and removal units which again reduces the plant's reliability. These changes are required since the sediment properties are not stable and depend on the quality of source water, season and water treatment technology.
This plant fails to provide for time coordination of the freezing and thawing processes due to possible accumulation of the frozen sediment on the surface of water in the melter. The transfer of heat from refrigerant to sediment in the melter proceeds with a convective heat transfer coefficient from the side of sediment not exceeding 100 W/m.sup.2 .multidot..degree.C., therefore the condensing heat exchanger should have an extended heat-exchanging surface which complicates the precipitation of solids from the sediment. Accumulation of solids from the sediment on the walls of the conical bottom of the vessel interferes with operation both of the melter and the sediment dehydration apparatus which tells on the reliability of the whole plant.